1. Field of the Invention
This invention relates generally to standing wave particle beam accelerators, and more particularly, to electron accelerators for generating x-ray and electron beams of different energies.
2. Background of the Invention
Standing wave particle beam accelerators have found wide usage in medical accelerators where the high energy particle beam is employed to generate x-rays. In this application, the beam energy and output dose-rate must be stable. It is also desirable that the energy of the particle beam be switchable readily and reliably to provide treatment beams of different energies to enable a range of dose-depth penetration during medical treatments.
Various techniques for controlling the beam energy have been employed. In U.S. Pat. No. 4,286,192 to Tanabe and Vaguine, the energy is controlled by reversing the accelerating fields in one part of the accelerator to decelerate the beam. In U.S. Pat. No. 4,382,208 to Meddaugh et al., the electromagnetic field distribution is changed in the switch-side cavity to control the fields applied to the adjacent resonator cavities. U.S. Pat. No. 4,746,839 to Kazusa and Yoneda discloses the use of two coupling cavities which are switched to control the acceleration fields.
Accelerators employing the previously described techniques can generally provide two to three different x-ray modalities (i.e., distinguished by clinically significant differences in energy levels) sufficient to meet treatment requirements. There would be a significant advantage however, both to the hospitals and the manufacturing process, to have an accelerator system capable of generating multiple high output x-ray modalities ranging over a factor of four to five in energy. From a manufacturing perspective, accelerators limited to 2 to 3 modalities are difficult and costly to implement. Currently, different modalities are configured by means of manufacturing different accelerator structures to provide different ranges of beam energies in order to meet energy requirements for different hospitals. As such, if a hospital changes its energy beam requirement, a different accelerator will have to be built. For the foregoing reason, there is a need for a standing wave electron accelerator capable of providing a range of energies that is broad enough to meet all hospital requirements. In addition, there is potential benefit in many medical procedures to have more than two levels of output x-ray energy to provide more sophisticated tailoring of dose-depth profile for treatment of cancer. As such, a standing wave particle beam accelerator which is capable of providing a plurality of levels of different output energy is desirable.